In current wireless communications systems, 5 MHz˜20 MHz radio bandwidths are typically used for up to 100 Mbps peak transmission rate. Much higher peak transmission rate is required for next generation wireless systems. For example, 1 gbps peak transmission rate is required by ITU-R for IMT-Advanced systems such as the 4th generation (“4G”) mobile communications systems. The current transmission technologies, however, are very difficult to perform 100 bps/Hz transmission spectrum efficiency. In the foreseeable next few years, only up to 15 bps/Hz transmission spectrum efficiency can be anticipated. Therefore, much wider radio bandwidths (i.e., at least 40 MHz) will be necessary for next generation wireless communications systems to achieve 1 Gbps peak transmission rate.
Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multiplexing protocol to perform high transmission rate over frequency selective channel without the disturbance from inter-carrier interference. There are two typical architectures to utilize much wider radio bandwidth for OFDM system. In a traditional OFDM system, a single radio frequency (RF) carrier is used to carry one wideband radio signal, and in an OFDM multi-carrier system, multiple RF carriers are used to carry multiple narrower band radio signals. The multi-carrier operation is also known as carrier aggregation. An OFDM multi-carrier system has various advantages as compared to a traditional OFDM system such as lower Peak to Average Power Ratio, easier backward compatibility, and more flexibility. Thus, OFDM multi-carrier wireless systems have become the baseline system architecture in IEEE 802.16m and LTE-Advanced draft standards to fulfill system requirements.
Scanning and handover are critical operations in wireless communications systems. FIG. 1 (Prior Art) is a message sequence diagram of a normal scanning procedure in a single-carrier wireless system. Before the initiation of the scanning procedure, a mobile station (MS) may receive parameters for neighbor base stations (BS#2 and BS#3) from serving base station BS#1. The MS then sends a scanning request to BS#1 and receives a scanning response back from BS#1. The MS then scans the neighbor BSs within the scheduled scanning period. Optionally, the MS may also perform initial ranging with the neighbor BSs. During the scanning operation, data communication is interrupted because of interleaved scanning intervals. In an autonomous scanning procedure, the MS simply performs scanning whenever it does not communicate data with its serving BS.
FIG. 2 (Prior Art) is a message sequence diagram of a normal handover procedure in a single-carrier wireless system. In either a BS-initiated or an MS-initiated handover operation, existing data path is disconnected from the serving base station (S-BS) after the MS transmits a handover indication message, and data communication remains interrupted while the MS performs network reentry to the target base station (T-BS) until a new data path is established with the T-BS. In a seamless handover procedure, if the T-BS is fully synchronized with the S-BS in both time and frequency domain, then the MS may proceed to data transmission immediately without downlink synchronization and initial ranging.
In OFDM multi-carrier wireless systems, it is desirable to have a comprehensive solution to facilitate multi-carrier scanning and handover operations such that data interruption caused by scanning and handover can be reduced.